Tunable ferromagnetic rod loop antenna



Aug. 16, 1966 G. SCHIEFER TUNABLE FERROMAGNETIC ROD LOOP ANTENNA Filed Jan. 18, 1963 FIGJ INVENTOR GERD SCHIEFER BY 11...). a

AGEN

United States Patent 3,267,478 TUNAELE FERRUMAGNETE RG1) L00? ANTENNA Gerd Schiefer, Aachen, Germany, assignor to North American Philips Company, Inc, New York, N.Y., a corporation of Deiawarc Fiied .lan. 13, 1963, er. No. 252,400 Claims priority, application Netherlands, Jan. 19, 1962, 273,756 15 Ciaims. (Cl. 343-788) The invention relates to a magnetic antenna, and more particularly to an improved device of this type adapted for use with frequencies exceeding 10 mc./s.

Magnetic antennas are known which comprise a cylindrical body of ferromagnetic material, for example ferrite, wherein the ratio of slenderness, i.e., the ratio between the length of the cylinder and its diameter, exceeds 5 to l. The cylindrical body in these devices is also provided with a coupling winding for connecting the antenna to a radio receiver, or the like.

Magnetic antennas, when compared with the more conventional electric dipoles, have the advantage that they are less sensitive to various radio and electrical interferences. Antennas of this type are usually mounted inside the radio receiver apparatus. However, magnetic antennas of small size have a very low radiation resistance so that difficulties arise when the antenna impedance must be matched to the impedance of the transmission line to which it is coupled or of the input stage of a radio receiver.

An object of this invention is to provide an improved magnetic antenna construction which provides a higher efficiency and sensitivity than conventional devices of this type.

Another object of this invention is to provide an improved magnetic antenna apparatus having a characteristic impedance which can be closely matched to the impedance of the transmission line to which it is coupled.

The problems involved in providing an antenna of the type described for the reception of frequency modulated waves in the broadcast band of about 100 mc./s. has not yet been adequately solved by any of the known prior art devices. These waves are usually horizontally polarized when radiated from the transmitter station. The use of a magnetic dipole antenna with a vertical axis has the additional advantage that it need not be rotatable, since it does not exhibit a directional effect in the horizontal plane. In contrast, a horizontal electric dipole has a marked directional effect, and in fact has two zero directions in which substantially no reception is possible.

The present invention provides an improvement in antennas of the type described. In accordance with the invention, the antenna comprises a cylindrical body of ferromagnetic material which is surrounded by a cylindrical sheath or sleeve of electrically conductive material, for example copper. This sheath is provided with a longitudinal gap across which there is connected a tuning capacitance distributed throughout its length. For optimum results, the length of the cylindrical sheath is preferably between 0.6 5 and 0.9 times the length of the cylindrical body of ferromagnetic material.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood from the following description considered in connection with the accompanying drawing, in which:

FIG. 1 is an elevation view of a magnetic antenna apparatus incorporating the principles of the present invention.

FIG. 2 is a plan view of the novel apparatus of FIG. 1.

FIG. 3 illustrates another preferred embodiment of the invention.

FIG. 4 shows a sectional view taken along the line A-B of FIG. 3.

FIG. 5 is a sectional view of an alternative embodiment of the invention.

The magnetic antenna shown in FIGS. l and 2 comprises a cylindrical core 1 composed of a ferromagnetic material, for example a ferrite. The core 1 is provided with a coupling winding 2, shown diagrammatically as having a pair of supply conductors 3 for coupling the antenna to the input stage of a radio receiver. The core 1 is surrounded by a sleeve or sheath 4 of electrically conductive material, for example of copper. The sleeve is also provided with a longitudinal gap extending the entire length of the sleeve. Sleeve 4 is slightly shorter than core 1. The opposite edges of the sleeve which form the gap 5 are capacitatively coupled with each other by means of a tuning capacitance 6 distributed over the length of the gap. This capacitance may consist, for example, as is shown in FIG. 1, of a plurality of capacitors 6' equidistantly spaced. Capacitance 6 preferably is given a value at which it resonates with the inductance of sleeve 4 at the midpoint of the operation wavelength range for which the antenna is intended for use. For example, if the apparatus is to be. used for the reception of the broadcast frequency modulation band which extends from 88 to 102 mc./s. the sleeve 4 and capacitors 6 are preferably tuned to approximately mc./s.

It has been found that the ratio of the length of the ferrite core 1 to its diameter must not be too small. The ratio is preferably higher than 5; the ratio 8 having been found to be a useful value in practice. Experimentation has also led to the discovery that in order to obtain a favorable overall effect, the length of the sleeve 4 should preferably lie between 0.65 and 0.9 times the length of the core 1. If the sleeve is longer, the receptive power or sensitivity. of the antenna decreases rapidly, since the core 1 is then effectively screened by the sleeve 4- from the radio waves.

It has been found that the provision of the resonating sleeve 4 greatly increases the sensitivity of a given magnetic rod antenna having a particular set of dimensions. If use is made of a ferrite core having a permeability of about 16 and a loss factor tan 6=about 0.006 and with a wavelength of 3 meters, the sensitivity of the receiver with the resonating sleeve was found to be about 5 times (7 db) that of a magnetic antenna with the same core, but without the sleeve. In both cases the bandwidth was about the same, i.e., about 1 mc./ s. This was to be expected, since the bandwidth is determined.

primarily by the magnetic losses alone. In both cases the coupling was such that the input impedance was 2409.

The improved effect of the antenna according to the invention might be accounted for by the fact that the conductive sleeve reduces the stray flux paths by preventing magnetic lines of force from emanating laterally from the bar. Therefore, the length of these lines of force is, on the average, greater than in the absence of a sleeve, and it follows that a substantially greater percentage of the flux is confined to the bar itself.

It has also been found that owing to the provision of the resonating sleeve, the energy coupled between the bar and the coupling winding is much greater than is the case where only the bar alone is used. In fact, even with the use of a single coupling turn surrounding the whole ferrite bar, an antenna impedance is obtained which exceeds the value desirable in practice. Normally, for example, use is made of symmetrical supply conductors having an impedance of 300 ohms or 240 ohms, or of coaxial supply conductors having an impedance of 75 ohms or 60 ohms. In order to adjust the antenna impedance to the value of the transmission line impedance, the coupling winding must enclose only a part of the total magnetic flux in the sleeve. This may be achieved by providing the ferrite bar with a transverse saw cut, in which the coupling winding is accommodated.

An advantageous embodiment is shown in FIG. 3, in which the ferrite body 1 consists of two separate portions 7 and 8, separated by an air gap 9, in which the coupling winding or pick-up coil is arranged. This construction has the added advantage that by the axial displacement of the two ferrite portions 7 and 8 relative to each other, the antenna tuning may be varied within given limits.

FIG. 4 is a cross sectional view taken along the lines A-B of FIG. 3. A pair of conductors 3 of a symmetrical transmission line are connected to points on the inner wall of the sheath 4. The cross-hatchedpart in the figure indicates the flux enclosed by the coupling winding and by enlarging or reducing this part the antenna impedance can be adjusted to any desired value within limits. FIG. 4 also shows how the tuning capacitance 5 may be formed by curved, opposite parts of the sheath 4. This construction has the advantage that it eliminates the need of a plurality of individual capacitors as shown in FIG. 1.

A particular advantage of the antenna according to the invention consists in the possibility of using an asymmetrical transmission line for coupling the antenna to the :receiver. Within the device shown in FIG. 5, the outer sheath of a coaxial transmission line is connected to the sleeve 4 at a point lying diametrically opposite the longitudinal gap 5. The inner conductor 11 is connected to a point at the inner wall of the sleeve 4. Here too, the antenna impedance can be varied by varying the volume enclosed by inner conductor 11. The tuning capacitance may also be formed by overlapping parts 12 and 13 of the sleeve 4, as shown in FIG 5. The supply conductors of the coupling winding may be taken out of the sleeve not only laterally, but also through an axial hole in the ferrite bar.

Although the particular form of the invention herein shown and described is a preferred embodiment of the invention, it is to be understood that various changes in the shape, size and arrangement of parts may be resorted to without departing from the true spirit of this invention or from the scope of the appended claims.

I claim:

1. A magnetic antenna for use over a predetermined radio frequency band comprising an elongated body of ferromagnetic material, a coupling winding mounted in magnetic coupling arrangement with said body, a sleeve of electrically conductive material having a longitudinal gap and arranged to surround said coupling winding and a substantial portion of said body, and means providing a tuning capacitance between the edges of said longitudinal gap.

2. A magnetic antenna for use with radio signals of a predetermined band of frequencies comprising a relatively thin elongated body of ferromagnetic material, a coupling winding mounted in magnetic coupling arrangement with said body, a sleeve of electrically conductive material having a longitudinal gap and arranged to surround said coupling winding and a portion of said body, and means providing a tuning capacitance distributed throughout the length of said gap and forming a resonant circuit with the inductance of said sleeve tuned to a given frequency within said band of signal frequencies.

3. A magnetic antenna for use over a given radio frequency band comprising a thin cylindrical body of ferromagnetic material, a coupling winding mounted in magnetic coupling arrangement with said body, a sleeve of electrically conductive material having a longitudinal gap and arranged to surround said coupling winding and a portion of said body, the length of said sleeve being between 65 and 90% Of the length of said cylindrical body,

and means providing a capacitance between the edges of said longitudinal gap which is substantially uniformly distributed throughout the length of said gap, said capacitance forming a resonant circuit with the inductance of said sleeve which is tuned to a given frequency within said radio frequency band.

4. A magnetic antenna comprising a thin cylindrical body of ferromagnetic material, the length of said body being at least five times its diameter, a coupling winding mounted in magnetic coupling arrangement with said body, a sleeve of electrically conductive material having a longitudinal gap and arranged to surround said coupling winding and a portion of said body, the length of said sleeve being between and of the length of said cylindrical body, and means providing a tuning capacitance between the edges of said longitudinal gap which forms a resonant circuit with the inductance of said sleeve.

5. Apparatus as described in claim 4 wherein said tuning capacitance means comprises a plurality of capacitors uniformly spaced along the length of said gap and connecting the opposed edges of said longitudinal gap so as to form a tuning capacitance uniformly distributed over the length of said gap.

6. Apparatus as described in claim 4 wherein said sleeve further comprises outwardly fianging edges along said longitudinal gap, said fianging edges coacting to form a substantially uniformly distributed tuning capacitance.

7. Apparatus as described in claim 4 wherein said coupling winding comprises a single turn of electrically conductive wire arranged around said cylindrical body.

8. A magnetic antenna comprising a relatively thin elongated body of ferromagnetic material having a transverse air gap located therein, a coupling winding mounted in said air gap in magnetic coupling arrangement with said ferromagnetic body, a sleeve of electrically conductive material having a longitudinal gap and arranged to surround said coupling Winding and a portion of said body, and means providing a capacitance between the edges of said sleeve gap which forms a resonant circuit with the inductance of said sleeve.

9. A magnetic antenna for use over a given radio frequency band comprising a relatively thin cylindrical body of ferromagnetic material having a transverse air gap located therein, a coupling winding mounted in said air gap in magnetic coupling arrangement with said ferromagnetic body, a sleeve of electrically conductive material having a longitudinal gap and arranged to surround said coupling winding and a portion of said body, the length of said sleeve being between 65% and 90% of the length of said cylindrical body, and a tuning capacitance distributed throughout the length of the sleeve gap which forms a resonant circuit with the inductance of said sleeve which is tuned to a given frequency within said radio frequency and.

10. A magnetic antenna comprising a relatively thin cylindrical body of ferromagnetic material having a transverse air gap located therein, a coupling winding mounted in said air gap in magnetic coupling arrangement with said ferromagnetic body, said coupling winding comprising a single turn of electrically conductive wire having a cross sectional area enclosed by said turn which is less than the cross sectional area of said cylindrical body, a sleeve of electrically conductive material having a longitudinal gap and arranged to surround said coupling wind ing and a portion of said body, and a tuning capacitance distributed throughout the length of said sleeve gap which forms a resonant circuit with the inductance of said sleeve.

11. A magnetic antenna for use over a given radio frequency band comprising a pair of relatively thin cylindrical bodies of ferromagnetic material, said bodies being axially aligned and spaced apart to form an air gap therebetween, eenrgy coupling means mounted in magnetic coupling arrangement with at least one of said ferromagnetic bodies, a sleeve of electrically conductive material having a longitudinal gap therein and arranged to su r-.

round said energy coupling means and a portion of said bodies, and means providing a tuning capacitance between the edges of said sleeve gap.

12. Apparatus as described in claim 11 wherein said tuning capacitance forms a resonant circuit with the sleeve inductance which is tuned to a given frequency within said radio frequency band and wherein at least one of said ferromagnetic bodies is axially movable so that the antenna tuning can be varied.

13. Apparauts as described in claim 11 wherein said energy coupling means comprises a pair of electrical conductors connected to internal points on said sleeve so as to form a closed circuit with a portion of said sleeve, said closed circuit enclosing a portion of the total magnetic flux within the sleeve.

14. Apparatus as described in claim 13 wherein said pair of electrical conductors form part of a symmetrical transmission line which carries the antenna energy and wherein the antenna impedance can be varied by varying the cross sectional area enclosed by said closed circuit.

15. A magnetic antenna comprising a pair of relatively thin cylindrical bodies of ferromagnetic material, said bodies being axially aligned and spaced apart to form an air gap therebetween, energy coupling means mounted in magnetic coupling arrangement with at least one of said ferromagnetic bodies, a sleeve of electrically conductive material having a longitudinal gap therein and arranged to surround a portion of said bodies, said energy coupling means comprising a coaxial transmission line having inner and outer conductors, said outer conductor being connected approximately to the midpoint along the length of said sleeve and diametrically opposite the longitudinal gap in said sleeve, said inner conductor being connected to a point on the inner wall of said sleeve so as to form a loop with a portion of said sleeve wall, said loop enclosing a portion only of the total magnetic lines of force enclosed by said sleeve, and means providing a capacitance between the edges of said sleeve gap which forms a resonant circuit with the inductance of said sleeve.

References Cited by the Examiner UNITED STATES PATENTS 2,611,867 9/1952 Alford 343-872 2,821,708 1/1958 Blancher 343-768 2,983,919 5/1961 Siukola 343-767 FOREIGN PATENTS 1,141,361 9/1957 France.

HERMAN KARL SAALBACH, Primary Examiner.

W. K. TAYLOR, E. LIEBERMAN, Assistant Examiners. 

1. A MAGNETIC ANTENNA FOR USE OVER A PREDETERMINED RADIO FREQUENCY BAND COMPRISING AN ELONGATED BODY OF FERROMAGNETIC MATERIAL, A COUPLING WINDING MOUNTED IN MAGNETIC COUPLING ARRANGEMENT WITH SAID BODY, A SLEEVE OF ELECTRICALLY CONDUCTIVE MATERIAL HAVING A LONGITUDINAL GAP AND ARRANGED TO SURROUND SAID COUPLING WINDING AND A SUBSTANTIAL PORTION OF SAID BODY, AND MEANS PROVIDING A TUNING CAPACITANCE BETWEEN THE EDGES OF SAID LONGITUDINAL GAP. 